Probing the polarized emission from the accretion-powered pulsar 4U 1907+09 with IXPE

Abstract

<p>We present observations of the accretion-powered X-ray pulsar 4U 1907+09 conducted with the Imaging X-ray Polarimetry Explorer, which has delivered the first high-quality polarization measurements of this source. 4U 1907+09 was observed twice during its brightest periods, close to the periastron. We observe a stronger polarization in the first observation, with a phase-averaged polarization degree (PD) of 6.0 +/- 1.6% and a polarization angle (PA) of 69° +/- 8°. The second observation provides weaker constraints on the polarimetric properties, PD = 2.2 +/- 1.6% and PA = 46° +/- 23° , as determined from the spectro-polarimetric analysis. Combining the data from the two observations results in PD = 3.7 +/- 1.1% and PA = 63° +/- 9° . We detect an energy-dependent PA in the phase-averaged analyses with a significance of 1.7<em>σ</em>. In the phase-resolved analyses, we observe a potential PA rotation of approximately 90° between adjacent energy bands (4-5 and 5-6 keV) within the single phase bin of 0.25-0.375. We also investigate the influence of short flares on the polarization properties of this source. The results suggest that flares do not significantly affect the energy-phase-dependent PA, implying that the pulsar's geometry remains stable during flare events.<br></p>